Hello, if you normally wear size 39, the ideal size for the Adidas Runfalcon 5 W model will also be 39. The mold is full size. However: if you have a narrow foot structure or you stand for a long time, size 40 may be more comfortable. If it is size 38, it will be too small, we definitely do not recommend it. So according to your current habit, you can choose size 39, if you want a little more comfort, you can choose size 40. If you need me to help with something different, KoraySpor is always with you.